FIELD OF THE INVENTION AND RELATED ART
The present invention relates to optical line tracers and in particular to those optical line tracers of the circular scanning, non-steering type shown, for example in U.S. Pat. No. 3,704,372 issued Nov. 28, 1972 to Robert E. Parker, et al; U.S. Pat. No. 3,727,120 issued Apr. 10, 1973 to George S. Jewell, et al; U.S. Pat. No. 3,860,862 issued Jan. 14, 1975 to William Dell, et al; U.S. Pat. No. 3,883,735 issued May 13, 1975 to Francis P. Murphy, et al. These tracers normally scan the pattern by reflecting a portion of the pattern from a mirror onto an optically sensitive device. The mirror is rotated to cause the portion of the pattern viewed to rotate about a centre and produce a circular scan. The signal produced by the optically sensitive device may then be processed and produce co-ordinate velocity signals which may be used to cause the tracer to follow the pattern at a constant tangential velocity.
All pattern followers must view the pattern at a point in advance of the actual axis of rotation or steering of the apparatus. Failure to provide sufficient advance will result either in system instability or in the inability of the pattern follower to trace a pattern which deviates from a straight line. The degree of advance is related to the accuracy of tracing and so a trade-off is required between the stability of the system, the accuracy of tracing and the velocity of movement of the system. These various factors establish the desired advance. It would be desirable to trace at high speed with minimum advance and thus maximum accuracy as long as the pattern did not deviate too far from a straight line and, on the other hand, to permit the tracer to negotiate rapid deviations. In order to do this it has been proposed in the past that the tracer slow down at corners or other deviations of the pattern to enable the tracer to negotiate the turns. In order to determine that a corner is about to occur it is necessary to view the pattern in advance of the actual axis of rotation location.